Asymmetric synthesis via chiral acetal templates. 7. Further studies on the cyanation reaction. The use of acetals derived from diols with one chiral center
✍ Scribed by Vanessa M.F. Choi; John D. Elliott; William S. Johnson
- Publisher
- Elsevier Science
- Year
- 1984
- Tongue
- French
- Weight
- 249 KB
- Volume
- 25
- Category
- Article
- ISSN
- 0040-4039
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✦ Synopsis
New exwnpZes of the cyanation reaction are described, including one that affords the cyanokydrin 8_, an intermediate for synthesizing pyretkroid insecticides. Also the reaction with acetals derived from S-1,3-butanediol has been examined. _ Recently we have shown that the Lewis acid catalyzed reaction of acetals (I_) derived from R,R-2,4-pentanediol,2'3 --with various nucleophilic partners (NuY) proceeds highly diastereoselectively to give adducts 2 in high yield. Removal of the chiral auxiliary from these adducts via the ketone 3 affords secondary hydroxy compounds 4 in high yield and generally in ee of 90% or over. Thus homoallylic alcohols have been produced from allyltrimethylsilane as the nucleophile;4 propargylic alcohols from silylacetylenic compounds;5 and cyanohydrins (as well as some derived a-hydroxy esters and B-amino alcohols) from cyanotrimethylsilane. ' These products all have the same absolute configuration at the newly developed chiral center, shown in formula 4 _. The method also lends itself to preparation of products of the opposite stereochemical series by use of the acetals (enantio-1) derived from S,z-2,4-pentanediol. 2 -Nu O-elimination NU acidic or basic method 1 2 3 4 It is not so clear that the model suggested for the transition state of the reaction of 5-membered ring acetals with nucleophiles is applicable to the 6-membered counterparts.4 We rather prefer the SN2-like transition state A which is stabilized by a lengthening of the 2,3 bond (process "a") of the ground state B with consequential relief of the relatively large7